A research team from the Institute of Sustainable Agriculture in Córdoba (IAS-CSIC) has developed a method to study with greater precision how certain bacteria interact with agricultural plastic waste. For the first time, they have differentiated the microbial communities that live directly on this material, which they have named plastic plane or plastiplane, from those that grow in the soil that sticks to it, the plastisphere. This advancement lays the groundwork for better understanding the impact of these residues on terrestrial ecosystems and opens the door to identifying microorganisms capable of degrading them.
Additionally, this work, funded by European funds from the SOPLAS project, framed within the Marie Skłodowska-Curie actions of the Horizon 2020 program, and in collaboration with the Ministry of University, Research and Innovation of the Andalusian Government, through the ‘Qualifica’ program, could be useful in designing strategies to decompose these wastes more effectively and sustainably, known as bioremediation. «When we talk about plastic pollution, we mainly think of environments like seas or rivers, and we hardly focus on soils, where there is also significant contamination,» highlights Giovana Macan, researcher at the Institute of Sustainable Agriculture in Córdoba and author of the study, to the Descubre Foundation, an organization dependent on the Ministry of University, Research and Innovation.
Another novelty compared to previous research was the collection of real conventional plastic samples, made from polyethylene, a material present in many everyday objects, such as bags, containers, or caps. The fragments came from various fields in Baza (Granada), an area characterized by intensive horticultural agriculture, where the use of plastic mulching is common. This type of sheet covers the soil to retain moisture, control temperature, and weeds, thus increasing crop yield. However, the sheets break down over time, generating both visible or macroplastics residues, as well as other particles smaller than five millimeters known as microplastics. These accumulate in the soil and are difficult to remove, which is why the United Nations Environment Programme considers it one of the most concerning contaminants of the 21st century, not only due to its persistence but also its environmental ubiquity, being detected in terrestrial ecosystems, aquatic environments, food, and even living organisms like the human body.
Plastisphere and plastiplane
The IAS-CSIC began addressing this issue with the SOPLAS training network, which covers 14 doctoral projects of young Europeans studying the interaction of plastic in agricultural soil, each with different techniques and objectives, but with mutual cooperation actions. Under this network, another study was presented at this center that tracked the spread of plastic during tillage for the first time.
Laboratory of Soil Microbiota Biology and Ecology.
Now, in the study entitled ‘Unravelling the plastisphere-soil and plasticplane microbiome of plastic mulch residues in agricultural soils’ and published in the journal Applied Soil Ecology, the experts have developed a protocol to separate two zones in the collected samples: the direct surface of the material, which they have named plastiplane, and the one with soil residues attached, the plastisphere. This distinction is crucial because both compartments present different conditions and can harbor distinct bacterial families. «We have extrapolated two concepts from microbiology: the rhizoplane, which corresponds to the root surface, where a more specialized microbial community resides, and the rhizosphere, which includes the surrounding soil influenced by the root, different from the rest of the terrain, favoring the growth of certain microorganisms,» explains Dr. Blanca Landa, principal researcher of the study at IAS.
The team designed a methodology based on sequential washes to extract the most adherent soil and sonication, a technique that uses ultrasonic waves to detach microorganisms directly attached to the plastic surface. With optical and scanning electron microscopes, which allow visualization of very small particles, they visually confirmed that both fractions had been separated correctly. For subsequent analysis, they combined traditional cultivation techniques with high-throughput genetic sequencing, which directly analyzes DNA to determine the complete composition of the microbial community. The results showed that the plastisphere contained a greater richness of bacteria, but the plastiplane also harbored a specific community, with some genera present in both compartments. Among them, some have been described as potential degraders of this contaminating material.
Future applications
In addition to identifying the present bacteria, the scientists isolated some specimens of interest for future research on bioremediation. «If these microorganisms survive on the plastic surface, it is very likely that they are using it as a nutrient source. If confirmed, we could grow them in the laboratory on different plastic substrates and observe if they are capable of partially or completely decomposing them,» suggests Macan.
Since it is based on real samples, the methodology can be adapted to other types of materials or agricultural conditions. In fact, the team has applied it in subsequent studies with fragments from blueberry fields in Huelva, and other experimental biodegradable plastics being developed at the Horticultural Institute of La Mayora (Málaga), made from cellulose or tomato residues.